State of the art controllers for fuel burners such as furnaces are now based on microprocessors which dramatically improve the control process. Nevertheless, it is still necessary to provide information as to the current operating state of the fuel burner. Among the most important of the state parameters is whether there is flame actually consuming the fuel being provided to the burner. The continued supply of fuel to the burner must be conditioned on the presence of flame, since if flame is not present and fuel is allowed to flow to the burner, the accumulation resulting can explode or asphyxiate, either one a potentially lethal event. Accordingly, it has been recognized for a long time in burner control technology that detection of flame is of paramount importance.
There are basically three kinds of flame detector elements. Perhaps the simplest is the so-called flame rod, which is simply a metal element insulated from the burner metal and located within the ionized particles forming the flame, and which forms with the burner metal a sort of diode element when flame is present. The diode action arises from the difference in the size of the flame rod compared to the burner itself. An AC potential applied between the flame rod and the burner metal causes DC current to be carried by the ionized particles generated by presence of a flame. By detecting presence of this DC current flow, it is possible to determine presence of flame. Because of the difference in sizes of the flame rod and the burner, the current flow is from the flame rod to the burner, meaning that presence of flame is signified by current flow into the flame rod signal conductor, placing its potential below ground voltage as represented by the burner.
A second type of flame detector circuit uses a phototube sensitive to ultraviolet radiation, and which produces a characteristic change in impedance indicating flame when such radiation is present. U.S. Pat. No. 5,194,728, patented on Mar. 16, 1993 by Scott Peterson, entitled "Fail-Safe UV Amplifier that is Compatible with Flame Current Digitizer Circuit", and having a common assignee with this application, uses the special characteristic of the ultraviolet-sensitive phototube to discriminate between actual presence of ultraviolet radiation and other causes of change in phototube impedance. The circuit of this application is noteworthy for the reason that it produces an output which simulates the output signal of the conventional flame rod sensor.
A third type, and the one with which the invention to be described deals, produces an output when infrared radiation produced by a flame impinges on an cadmium sulfide or other type of detector tube whose impedance drops in response to the radiation. Each of these sensors produces an output requiring substantial processing by special circuitry before a signal indicating presence and absence of flame and which is suitable to be an input to a microprocessor is generated. The circuitry which converts the flame detector signal to a signal suitable for use by the controller is referred to as a flame amplifier and its output as a flame present signal, or more simply, a flame signal In the case of infrared sensing, the characteristic of the signal which reliably indicates flame is known to be the presence of a 5 to 15 hz. flicker. This flicker is a natural result of combustion of the hydrocarbon fuels in common use, and many infrared sensor circuits use this flicker as a basis for indicating presence of flame. The flicker is only one component of the total infrared radiation output of the flame however, and a minor one at that, so those infrared sensor circuits relying on the flame flicker to distinguish between presence and absence of flame must carefully separate the flicker component of the infrared sensor from what is noise for the purposes of flame detection.
A flame rod amplifier circuit designed to operate with a positive DC power supply adds a measure of reliability to its operation by interfacing with a flame rod sensor whose output is a negative current, i.e., one whose current flows into the sensor from the flame amplifier. The extra measure of reliability arises from the fact that any leakage current within the flame amplifier cannot masquerade as the negative current flow forming the flame rod output. Any leakage current in a flame amplifier powered by positive voltage will almost invariably be positive, and thus not likely to be interpreted as the negative flame rod sensor output. A pending US patent application which covers a flame amplifier circuit embodying these concepts is titled Fail-Safe Condition Sensing Circuit, was filed on Oct. 28, 1991 with Ser. No. 07/783,950, and has a common inventor and assignee with this application.
The most efficient way to implement this flame rod amplifier is as a special purpose microcircuit. Because of this implementation, returns to scale are particularly high, meaning that the unit cost drops substantially with increases in the number of individual circuits produced. Accordingly, it is very advantageous for this flame rod amplifier to be compatible with not only the flame rod detector, but also with the UV and IR detectors. However, the power required to drive the UV and IR detectors is different from that required for flame rod detectors, and the output signal of each has substantially different characteristics from the other two. Accordingly, it is not possible to simply replace the flame rod detector with either a UV tube or infrared cell flame detector. Instead, the interface circuit described above for the UV tube, or the interface circuit of which the invention to be described forms a part, is necessary to provide a signal output compatible with a detector intended for use with a flame rod detector.
There are any number of circuits now known which are intended to detect presence of flame by sensing presence of the 5-15 hz. frequency component in the infrared spectrum. For example, recently issued U.S. Pat. No. 5,073,769 discloses apparatus for detecting indicative of flame by us of the discrete Fourier transform.